Guizhouichthyosaurus

Guizhouichthyosaurus ("Guizhou fish lizard") is an extinct genus of Ichthyosaur which existed during the lower Carnian stage of the Late Triassic in southwest China. The genus contains the single species Guizhouichthyosaurus tangae.[1] It had been referred to the genus Shastasaurus in the past, but later study showed that it is likely distinct, since it lacks the characteristically short snout of that genus. The ichthyosaurs Cymbospondylus asiaticus, named in 2002, and Panjiangsaurus epicharis, named in 2003, are junior synonyms of this genus.[2]

Guizhouichthyosaurus
Temporal range: Middle Triassic,
Fossils of Guizhouichthyosaurus tangae in Geological Museum of Guizhou
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: †Ichthyosauria
Genus: †Guizhouichthyosaurus
Species:
†G. tangae
Binomial name
†Guizhouichthyosaurus tangae
Cao & Luo, 2000 (type)
Synonyms

History of research

Guizhouichthyosaurus tangae was one of three new genera and species of ichthyosaurs from the Falang Formation[3] of China named by Y. Cao and Y. Luo in 2000.[1] G. tangae was named based on Gmr 009, a well-preserved specimen, although it is missing the hips, a hindflipper, and much of its tail.[4] Li Chun and You Hai-Lu named a new species of Cymbospondylus, C. asiaticus, from the same region in 2002 based on two nearly complete skulls. They noted that this was the first time that Cymbospondylus had ever been reported from outside North America and Europe, in addition to the Middle Triassic.[5] In 2003, another ichthyosaur, Panjiangsaurus epicharis, was named by X. H. Chen and L. Cheng, based on the nearly perfectly-preserved specimen TR 00001.[6]

A study of the cranial anatomy of Guizhouichthyosaurus was conducted by Michael Maisch and colleagues in 2006. They described multiple specimens; GNG dq-46, a good skull discovered by farmers at Wolonggang in 1998, then sent to the Guanling National Geopark of Fossil Biota where it was prepared by Jin-ZhaoDing and Da-Peng Zhang; GNG dp-22, a partial skull mixed up with the remains of other ichthyosaurs; and GNG D-41, a complete, though incompletely prepared, skeleton.[1] Pan Xinru and colleagues restudied Guizhouichthyosaurus and published their results in another 2006 paper, in which they described GNG D-41 in more detail. They found Panjiangsaurus epicharis and Cymbopsondylus asiaticus to be junior synonyms of G. tangae.[7]

In 2009, Shang Qing-Hua and Li Chun described a new specimen of Guizhouichthyosaurus, IVPP V 11853, a nearly complete articulated skeleton, missing only some parts of the flippers. They further described the anatomy of Guizhouichthyosaurus, using this new specimen to provide information on the shoulders, hips, and tail, which until then had been poorly known. However, they considered Guizhouichthyosaurus to be so similar to Shastasaurus that they synonymized it with that genus, although they kept S. tangae as a distinct species. They concurred with previous studies that Panjiangsaurus epicharis and Cymbospondylus asiaticus were junior synonyms of this species.[4]

However, in 2010, Maisch provisionally accepted Guizhouichthyosaurus as a distinct genus. While he considered the two genera to be quite similar, he noted that the majority of the traits that had been used to synonymize it with Shastasaurus were ancestral characteristics of that group of Triassic ichthyosaurs instead of novel evolutionary changes, and stated that further research was still needed before the two could be synonymized.[8] A 2011 paper by P. Martin Sander and colleagues considered Guizhouichthyosaurus to be quite distinct from Shastasaurus,[2] and while Shang and Li again used the name Shastasaurus tangae in a 2013 paper,[9] Guizhouichthyosaurus has since been accepted as valid and different from Shastasaurus.[10][11][3][12]

Callawayia wolonggangense

In 2010, Maisch moved the species Callawayia wolonggangense to Guizhouichthyosaurus, as G. wolonggangense, noting that it was distinctly different from Callawayia. He considered the characteristics used to differentiate G. wolonggangense unconvincing, however, and that this species was probably just a junior synonym of G. tangae. He still maintained it as provisionally valid though, as detailed investigation had not yet been done.[8]

In 2016, Ji and colleagues found no characteristics uniting "C." wolonggangense and the type species of Callawayia, C. neoscapularis, and thus rejected the assignment of the former species to this genus. However, they found no traits unambiguously linking it to Guizhouichthyosaurus either, thus also rejected its assignment to that genus.[10] Further studies have variably referred to the species as "C." wolonggangense or G. wolonggangense, but phylogenetic analyses have generally found results similar to those of Ji and colleagues, where the species is not found to be the sister taxon of the type species of either genus.[12][11][13][14]

Description

Guizhouichthyosaurus has been described as moderate to very large in size for an ichthyosaur.[4] XNGM-WS-50-R4, a nearly complete skeleton assigned to Guizhouichthyosaurus sp. measures 4.8 meters long.[3] As for specimens of G. tangae, the total length of the nearly complete skeleton IVPP V 11853 exceeds 5.2 meters,[4] while TR 00001 (the holotype of Panjiangsaurus) measures 5.4 meters long[6] and the complete skeleton GNP-d41 measures more than 6 meters long.[7] Additionally, specimens of Guizhouichthyosaurus reaching around 7 meters in total length have also been reported.[3]

Skull
Skull of Guizhouichthyosaurus sp., XNGM-WS-50-R4, with a close-up of the teeth

Guizhouichthyosaurus has a long, powerful snout.[4][10] An extensive groove is present in front of each external naris (nostril opening).[10] The premaxillae (front upper tooth-bearing bones) do not extend beneath the external nares. However, a spear-shaped projection of each premaxilla forms most of the upper rim of each external naris. The external nares are oval-shaped and small. The maxillae (back upper tooth-bearing bones) extend quite far forwards. They also bear long, rearwards extensions that almost touch the prefrontals (a pair of skull roof bones). The nasals (a pair of skull roof bones) extend as far back as to be level with the middle of the orbits (eye sockets), where they contact another pair of skull roof bones, the postfrontals. The orbits of Guizhouichthyosaurus are distinctively shaped, having convex front, straight rear and lower, and weakly[7] concave upper margins. The front thirds of their upper borders are formed by the prefrontals, which are small in Guizhouichthyosaurus.[1]

The portion of the skull located behind the orbits is rather short, not much more than half of the orbital length in adults.[4][1] There is a large, triangular indentation behind the jugals, the bones that form the lower rims of the orbits.[1][7] The contact between the jugals and quadratojugals (paired bones towards the back of the skull) is hidden on the outside of the skull by two other pairs of bones, the postorbitals and squamosals. The pineal foramen (an opening on top of the skull) is flanked by sharp ridges on the frontals and parietals (two pairs of skull roof bones).[1] The low, thin,[1] well-developed saggital crest on the parietals splits in two at its front and rear ends. A reasonably prominent flat shelf is present in the front regions of the depressions surrounding each temporal fenestra (openings on top of the skull).[4] The frontals form part of these shelves, but do not reach the fenestrae,[1] with the rest of the shelves formed by the postfrontals and parietals.[7] The front half of the outer edges of the fenestrae are formed by the postorbitals internally, but these bones are excluded from the exterior surface by the contacts between two pairs of skull roof bones, the postfrontals and supratemporals.[1] These bones, in addition to the parietals, form the exterior rim of the temporal fenestrae.[7]

The pterygoids (rear pair of palatal bones) lack prominent outwardly-directed projections. The space between the pterygoids is large. The surangulars (upper rear lower jaw bones) bear deep depressions that are confluent with depressions on the dentaries (lower tooth-bearing bones). The teeth of Guizhouichthyosaurus are conical, have somewhat blunted tips, and are set into sockets. They have 3 to 5 very fine vertical ridges, but lack cutting edges.[1][7]

Postcranial skeleton

There are about 65 vertebrae in front of the hips in Guizhouichthyosaurus, followed by 2 hip vertebrae. The tail of Guizhouichthyosaurus is elongate, containing around 170 vertebrae, and is abruptly bent downwards near its end. The wide scapulae (shoulder blades) are asymmetrical,[7] sickle-shaped, and lack constricted bases. The front edges of their blades are smooth and bow forwards, and do not have hook-like projections.[4] The coracoids (another pair of shoulder bones) have prominent indentations in their front edges.[7] The interclavicle (a shoulder bone between the collarbones) is shaped like the letter T. The central, lower projection of it is very small and narrow. The humeri (upper arm bones) have straight rear margins.[4] The inner edges of the radii and ulnae (lower arm bones) are concave, leaving an opening between the two bones in each flipper. This is also seen in the lower leg bones, the tibiae and fibulae.[7] An additional carpal (wrist bone) is present beneath each ulnare of Guizhouichthyosaurus. Unusually, the phalanges (digit bones) towards the tip of the digit are much larger than those closer to the base of digit II in the hindfins of some specimens.[10][9]

Paleobiology

A 2013 study by Shang and Li found that all specimens of Guizhouichthyosaurus with sufficient good preservation could be divided into two morphotypes, though they were all sufficiently similar to belong to the same species. The morphotypes were named "type A" and "type B." Type A is characterized by long, narrow hindflippers, with the upper elements of digit II being reduced in size and no additional digit in front of it present. Sometimes there is an additional digit behind the main digits of the hindfin, although there usually is not. Type B is characterized by broader hindflippers without the size reduction of the upper digit II elements, as well an additional digit in front of and another behind the main ones. The skulls of the two types also differ, with type A having a more robust skull, a shorter snout, and a wider angle between the two halves of the sagittal crest where it splits at the front than type B. Additionally, the temporal fenestrae are more than twice as long as wide in type B but less than twice as long as wide in type A. Since there was no significant difference in skull or body size between the two types, Shang and Li rejected that the differences were due to growth, and instead considered that they most likely represented differences between males and females. Since members of type A have proportionately longer trunks and shorter tails than type B, the researchers considered type A to be the females, although no pregnant specimens are known to confirm this. In modern lizards, the larger trunks of females increase the amount of room for eggs. While as an ichthyopterygian, Guizhouichthyosaurus would have been a live-bearer, a similar trend in trunk-tail proportions is also seen in another group of vivaporous marine reptiles, the pachypleurosaurs.[9][15]

In 2019, Susana Gutarra and colleagues studied the energy demands of swimming in ichthyosauriforms using computational fluid dynamics and models of nine taxa, one of which was Guizhouichthyosaurus. They found that the body shapes of the different ichthyosauriforms, when scaled to the same size, all resulted in similar net energy costs and drag coefficients. Through their simulations, the researchers found that the water would have stagnated around the snouts of the ichthyosauriforms, then flowed more quickly across their bodies before slowing down in the animal's wake. Guizhouichthyosaurus, in addition to Chaohusaurus, produced more drag than the other taxa, due to their large flippers. However, when the models were scaled to the actual size of the animals, it became clear that ichthyosauriforms became increasingly efficient swimmers in their first 25 million years of evolution, largely due to body size. Additionally, while Triassic ichthyosaurs such as Guizhouichthyosaurus would have used anguilliform (eel-like) locomotion, later ichthyosaurs switched to the more efficient carangiform (cod-like) and thunniform (tuna-like) swimming styles. This switch likely explains why these later ichthyosaurs had deeper bodies.[16]

Diet and feeding

TR 00001 preserves significantly more than 100 gastroliths in its stomach region. These stones vary in size from .6-3.7 centimeters in diameter and generally are rather round. These gastroliths were preserved in two clusters, however Long and colleagues doubted that Guizhouichthyosaurus would have been stored them in two different regions, a configuration seen in no other vertebrate. Instead, they speculated that the clusters formed after death, or even that some gastroliths might have killed the animal after moving into and clogging its intestines. As the gastroliths did not resemble the surrounding rock, the scientists concluded that Guizhouichthyosaurus must have obtained them somewhere else, perhaps near a beach. While the function of gastroliths has been interpreted by some as buoyancy control, this is controversial, and Long and colleagues found the roughly 1 kilogram of stones to be insufficient to usefully serve as ballast for an animal potentially weighing as much as a ton. Since no other specimen of Guizhouichthyosaurus has gastroliths, they considered it most likely that the stones in TR 00001 were swallowed accidentally when attacking prey near the seafloor.[6]

XNGM-WS-50-R4 (topmost), with stomach contents shown below

The absence of cutting edges on its teeth and their relatively small size would seem to indicate that Guizhouichthyosaurus did not take large prey. The prey preference of marine reptiles can be predicted based on tooth morphology, and can be divided into a variety of feeding guilds. Based on tooth shape, Guizhouichthyosaurus falls into the smash guild, with teeth suited to grasp soft prey such as cephalopods. However, this prediction is contradicted by XNGM-WS-50-R4, a specimen of Guizhouichthyosaurus sp. described by Da-Yong Jiang and colleagues in 2020. This specimen, excavated in 2010, preserved a partially articulated trunk of the thalattosaur Xinpusaurus in its stomach region. They estimated that the thalattosaur would have been 4 metres (13 ft) long when complete, compared to the 4.8 meter length of the ichthyosaur, and about one seventh of the latter's mass. A 2 metres (6.6 ft) long tail of Xinpusaurus was found 23 metres (75 ft) away, probably coming from the ingested individual. Due to its completeness and the scarcity of marine carrion, Jiang and colleagues considered it most likely that the thalattosaur was killed by the ichthyosaur, rather than scavenged. The good preservation of the thalattosaur also indicated that the ichthyosaur probably died soon after eating it. Jiang and colleagues noted that while teeth suited for grasping are useful in procuring cephalopods, they can also be used to hold prey underwater until it drowns, as done by crocodiles.[3]

To detach the prey's head and tail, the researchers hypothesized that Guizhouichthyosaurus would have torn them off using twists of its head and the inertia of the thalattosaur's body, as done by orcas and crocodiles, which, like Guizhouichthyosaurus, lack cutting teeth. The thalattosaur's trunk may have been swallowed in as many as four pieces, but it seems must likely that it was swallowed whole. Swallowing could have been accomplished via inertia and the use of gravity above the water, and while Guizhouichthyosaurus couldn't widen its skull via cranial kinesis, its small shoulder girdles would have permitted large items to pass between them. The good preservation of its stomach contents indicates that the ichthyosaur died soon after eating the thalattosaur. The authors noted that Guizhouichthyosaurus appeared to be an apex predator, and predation upon large animals was likely more common among marine reptiles than previously thought.[3]

See also

References
  1. Maisch, M. W.; Pan, X. R.; Sun, Z. Y.; Cai, T.; Zhang, D. P.; Xie, J. L. (2006). "Cranial osteology of Guizhouichthyosaurus tangae (Reptilia: Ichthyosauria) from the Upper Triassic of China". Journal of Vertebrate Paleontology. 26 (3): 588–597. doi:10.1671/0272-4634(2006)26[588:COOGTR]2.0.CO;2.
  2. Sander, P.M.; Chen, X.; Cheng, L.; Wang, X. (2011). "Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic Diversification of Suction Feeding Ichthyosaurs". PLOS ONE. 6 (5): e19480. Bibcode:2011PLoSO...619480S. doi:10.1371/journal.pone.0019480. PMC 3100301. PMID 21625429.
  3. Jiang, D. Y.; Motani, R.; Tintori, A.; Rieppel, O.; Ji, C.; Zhou, M.; Wang, X.; Lu, H.; Li, Z. G. (2020). "Evidence supporting predation of 4-m marine reptile by Triassic megapredator". iScience. 23 (9): 101347. Bibcode:2020iSci...23j1347J. doi:10.1016/j.isci.2020.101347. PMC 7520894. PMID 32822565.
  4. Shang, Q. H.; Li, C. (2009). "On the occurrence of the ichthyosaur Shastasaurus in the Guanling biota (Late Triassic), Guizhou, China" (PDF). Vertebrata PalAsiatica. 47 (3): 178–193.
  5. Li, C.; You, H. L. (2002). "Cymbospondylus from the Upper Triassic of Guizhou, China" (PDF). Vertebrata PalAsiatica. 40: 9–16.
  6. Long, C.; Wings, O.; Xiaohong, C.; Sander, P. M. (2006). "Gastroliths in the Triassic ichthyosaur Panjiangsaurus from China". Journal of Paleontology. 80 (3): 583–588. CiteSeerX 10.1.1.898.7202. doi:10.1666/0022-3360(2006)80[583:GITTIP]2.0.CO;2.
  7. Pan, X.; Jiang, D.; Sun, Z.; Cai, T.; Zhang, D.; Xie, J. (2006). "Discussion on Guizhouichthyosaurus tangae Cao and Luo in Yin et al., 2000 (Reptilia, Ichthyosauria) from the late triassic of Guanling County, Guizhou". Acta Scientiarum Naturalium Universitatis Pekinensis. 42: 697–703.
  8. Maisch, M. W. (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art" (PDF). Palaeodiversity. 3: 151–214.
  9. Shang, Q. H.; Li, C. (2013). "On the sexual dimorphism of Shastasaurus tangae (Reptilia: Ichthyosauria) from the Triassic Guanling Biota, China" (PDF). Vertebrata PalAsiatica. 51 (4): 253–264.
  10. Ji, C.; Jiang, D. Y.; Motani, R.; Rieppel, O.; Hao, W. C.; Sun, Z. Y. (2016). "Phylogeny of the Ichthyopterygia incorporating recent discoveries from South China". Journal of Vertebrate Paleontology. 36 (1): e1025956. doi:10.1080/02724634.2015.1025956. S2CID 85621052.
  11. Moon, B. C. (2019). "A new phylogeny of ichthyosaurs (Reptilia: Diapsida)" (PDF). Journal of Systematic Palaeontology. 17 (2): 129–155. doi:10.1080/14772019.2017.1394922. hdl:1983/463e9f78-10b7-4262-9643-0454b4aa7763. S2CID 90912678.
  12. Bindellini, G.; Wolniewicz, A.S.; Miedema, F.; Scheyer, T.M.; Dal Sasso, C. (2021). "Cranial anatomy of Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications". PeerJ. 9: e11179. doi:10.7717/peerj.11179. PMC 8106916. PMID 33996277.
  13. Maxwell, E. E.; Cortés, D. (2020). "A revision of the Early Jurassic ichthyosaur Hauffiopteryx (Reptilia: Ichthyosauria), and description of a new species from Southwestern Germany". Palaeontologia Electronica. 23: 1–43.
  14. Yin, Y. L.; Ji, C.; Zhou, M. (2021). "The anatomy of the palate in Early Triassic Chaohusaurus brevifemoralis (Reptilia: Ichthyosauriformes) based on digital reconstruction". PeerJ. 9: e11727. doi:10.7717/peerj.11727. PMC 8269639. PMID 34268013.
  15. Cheng, Y. N.; Wu, X. C.; Ji, Q. (2004). "Triassic marine reptiles gave birth to live young". Nature. 432 (7015): 383–386. Bibcode:2004Natur.432..383C. doi:10.1038/nature03050. PMID 15549103. S2CID 4391810.
  16. Gutarra, S.; Moon, B. C.; Rahman, I. A.; Palmer, C.; Lautenschlager, S.; Brimacombe, A. J.; Benton, M. J. (2019). "Effects of body plan evolution on the hydrodynamic drag and energy requirements of swimming in ichthyosaurs". Proceedings of the Royal Society B. 286 (1898): 20182786. doi:10.1098/rspb.2018.2786. PMC 6458325. PMID 30836867.
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